WordPress ajax function parameter value not being passed

First of all you should read the codex on AJAX_in_Plugins

Secondly you should look at wp_localize_script to get the value for the admin-ajax url to your javascript.

$data = array(
    'ajax_url' => admin_url( 'admin-ajax.php' )
);

wp_localize_script( 'ajax-script', 'ajax_object', $data );

In your javascript, you are then meant to reference the localized data

jQuery(".selectbox").change(function(){
    var id = this.id;

    // do a POST ajax call
    $.ajax({
        type: "POST",
        url: ajax_object.ajax_url,
        data: ({
            "action": "get-mata-value",
            "metakey": id
        }),
        success:  function( response ) {

        }
    });
});